1. Field of the Invention
The present invention relates to an image processing apparatus capable of eliminating noise in image signals.
2. Related Background Art
An image processing apparatus is known which processes image data read by a line sensor made up of charge coupled devices (CCDs) or contact sensor (CS), with the processed image data being printed by a printer. Multi-value signals of an image read by a line sensor are converted into density signals which are binarized by a pseudo half-tone process using an error distribution method, a dither method, or the like, and are printed by a printer.
Such a conventional apparatus uses a non-linear density-luminance conversion table which is not linear as shown in FIG. 2. Therefore, for example, a change in luminance level by one step at a low luminance level corresponds to M steps of the density level.
If noise is contained in a luminance signal read by a line sensor, the noise is emphasized at a low luminance level, i.e., at a high density level. Even one step of the luminance level may produce a conspicuous white or black line on an output image.
Smoothing filters, median filters and the like are known as a means for eliminating such noise. However, use of these filters eliminates noise on the one hand, and lowers the resolution of an image on the other hand.